Vit;
Our 2005 Horizon is a sister to yours. We purchased it used in 2011. We had a similar problem with the passenger side slide except it was the front slide mechanism that was making noise. I had some other issues that needed attention also.
We live about 6 hours from the Winnebago Factory at Forest City, IA. I made an appointment with WGO for factory service at their shop. I dropped it off and told them to call me when it was ready for pickup. they called me 2 or 3 days later and told me the Horizon was ready for pickup.
Everything I had on my list was taken care of to my satisfaction. The slide is not perfect but is about 70% better. It now only rattles on severe choppy roads along with several other rattles that appear. I highly recommend WGO factory if you are, or going to be, in a reasonable range. Nobody knows your motorhome better than they do.
A friend once told me to take my cottage and put it on the bed of a truck and drive down the road at 65 MPH and see how many rattles I have.
Bottom line I think your rattle can be much improved but that scissors mechanism needs some attention from some one with experience with it.
Don